While generally useful on their own inherent merits, the physical properties of styrene polymer-thermoplastic elastomer polyblend systems are considerably improved when an optimum level of cross-linking is achieved in the elastomeric constituent rubbery portion of the thermoplastic elastomer. This is particularly advantageous and practical when the styrene polymer constitutent of the polyblend is general purpose homopolystyrene.
The styrene polymers employed are generally polymers and copolymers of alkenyl aromatic monomers of the Formula: EQU CH.sub.2 =CGAr, (I)
wherein G is selected from the group consisting of hydrogen and methyl and Ar is an aromatic radical, including various alkyl and halo-ring-substituted aromatic units of from 6 to 10 carbon atoms. Styrene is ordinarily the most advantageous and oftentimes preferred species of the Formula (I) monomers to utilize. Others that are frequently quite satisfactory include: .alpha.-methyl styrene vinyl toluene; vinyl naphthalene; the dimethyl styrenes, t-butyl styrene; the several chlorstyrenes such as the mono- and dichloro-variants; the several bromostyrenes such as the mono- and dibromo-varients; and the like.
Copolymeric styrene polymers can be copolymerizates of one or more Formula (I) monomers, particularly styrene, with one or mixtures of other addition-polymerizable monoethylenically unsaturated comonomers that are copolymerizable with St including, by way of illustration and not limitation; acrylonitrile and methacrylonitrile; vinyl chloride and other vinyl halides; vinylidene chloride; acrylic acid and its addition-polymerizable esters; methacrylic acid and its addition-polymerizable esters; various vinyl organic esters such as vinyl acetate, vinyl propionate; and the like.
The styrene polymers utilized may also be the rubber-modified interpolymerized products of graftable pre-formed elastomers and monomers of the Formula (I). Typical of these are the so-called high impact polystyrenes. When use is made for the styrene polymers of rubber-modified, impact grade plastics products, it is customary for them to be prepared by incorporation in the composition of from, say, 1-20 wt. percent, of an unsaturated, graft-copolymerizable stock of natural or synthetic rubbery elastomers (as hereinafter more fully described) for interpolymerization with the monoethylenically-unsaturated monomer in the reaction mass; all according to established procedures. The modifying rubber in current vogue is polybutadiene or a polybutadiene derivative; although, if desired, natural rubbers may be employed as may styrene/butadiene polymers, for example, of the well-known "GRS"-type, polyether elastomers, and the like.
It is of general good advantage when copolymeric styrene polymers are employed for at least about 60 percent by weight, based on copolymer weight, of Formula (I) monomer(s) that are copolymerizable with styrene to be copolymerized in the polymer molecule. More advantageously, this is at least about 80 weight percent, with the balance of copolymerized ingredients being desired comonomer(s) that are copolymerizable with styrene.
An almost invariable and desirable characteristic of thermoplastic elastomers is their inherent combination of the natural flexibility and impact resistance of rubbers with the normally-usual strength and easy-processability of thermoplastics, coupled with features of frictional properties and hardness that are generally intermediate those of conventional rubbers and thermoplastics.
Generally, the thermoplastic elastomers may be characterized as rubbery or elastomeric block copolymers which, sometimes, are even in at least approximate if not actual graft copolymer form. They are, insofar as concerns the presently-contemplated polyblends, various sorts and arrangements of an elastomeric center or other possible backbone or substrate constituent to and upon which are attahced the end or otherwise connected blocks of interpolymerized styrene polymer units. In all cases, in order for an adequate inherent potential for cross-linkability to exist, the thermoplastic elastomers that are utilized must contain and exhibit a greater or lesser extent or degree of unsaturation therein.
Most, if not literally all, of the presently known varieties of thermoplastic elastomers are made by ionic, generally anionic, solution polymerization using an organometallic catalyst, such as sec.-butyl-lithium, n-butyl-lithium or the like or equivalent catalysts, as explained in Reference Number 12 (i.e., "Ref. 12") in the following "LISTING OF REFERENCES" Section of this Specification. Refs. 1 and 2 also deal with this.
Typical architecture(s) of thermoplastic elastomers are represented by the structures wherein IPSP represents a block of interpolymerized styrene polymer and EL represents an elastomeric segment: ##STR1## and so on and so forth, all wherein "n" is an integer which, usually, is 1 but can alternatively depend in numerical value on the particular molecular weight (generally a weight average measurement--i.e., "Mw") or chain length of given interconnected EL units in the instances when they are ultimately so joined or formed.
Structure (S I) is quite common, being represented by that commercial variety available from "THE GENERAL TIRE AND RUBBER COMPANY" made from polystyrene and polybutadiene in the block copolymer form IPSP-El-IPSP containing about 40 weight percent polystyrene and having a weight average molecular weight of about 550,000 (Ref. 6). Structures (S III) through (S V), inclusive, are at least by analogy more or less in the nature of graft copolymers. Structures (S IV) and (S V) are oftentimes referred to as "star-blocks" or "radial-blocks". A good example of a Structure (S V) star-block is that obtainable under the trade-designation "SOLPRENE" (Reg. .TM.), as described in Ref. 9. This is a radical block (IPSP).sub.4 -EL of varying polymerized styrene- to polybutadiene ratio and composition in differing molecular weight products. "KRATON G" (Reg. .TM.) is explained in Refs. 10 and 11 and typifies a commercially available Structure (I) material which is a styrene polymer hydrogenated-polybutadiene-styrenepolymer triblock of varying styrene to butadiene ratio polymerized therein, composition including mineral oil content. Structure (S VI) diblock copolymers often have what is referred to as a "tapered" interpolymerized construction of varying molecular weight and styrene to butadiene ratio.
The elastomeric blocks, often referred to as being the "soft" ones in styrene polymer-elastomer interpolymers, provide the rubbery properties to the interpolymer. The styrene polymer blocks, often referred to as being the "hard" ones in the subject interpolymers, tend to associate or conglomerate into glassy domains. These effectively function as "cross-links", at least insofar as restricting the free movement of the macromolecular thermoplastic elastomer chains is concerned. The styrene polymer blocks also give the product at least the bulk of its tensile strength. The styrene polymer block domains tend to disappear when softened by heat; re-forming when the interpolymer product is cooled. This, advantageously, allows processing and fabrication of the material according to the various techniques and procedures customarily followed for normal thermoplastics.
Frequently, the overall ratio of "hard" styrene polymer blocks to "soft" elastomer blocks in the thermoplastic elastomer structure is about 2:1 by respective chain(s) proportion reckoning. This is particularly so in strictly and somewhat classic types of block copolymers represented by the Structure (S I) and the more or less graft styles represented by Structures (S II) and (S III). It may also apply to many interpolymers of the (S IV), (S V), and even (S VI) Structures. Useful materials can be comprised of as little as about 20-25 weight percent or so of the elastomer constituent. Often, however, this elastomer content may be on the order of at least 45-50 weight percent and even greater.
The elastomer utilized for preparation of the thermoplastic elastomer may be selected from a wide variety of generally sulfur-vulcanizable materials. It can, for example, be natural rubber such as Hevea Brasiliensis. Much more often, however, it is a conjugated diolefine homopolymer synthetic rubber or elastomeric inter-, or co-polymer composition of between about 25 and about 90 weight percent of a 1,3-diene of the Formula: EQU H.sub.2 C:CR--CH:CH.sub.2, (II)
wherein R is selected from the group consisting of hydrogen, chlorine and methyl radicals.
Such conjugated diolefine polymer synthetic rubbers are polymers of: butadienes-1,3, e.g., butadiene-1,3; isoprene; 2,3-dimethylbutadiene-1,3; and copolymers of mixtures thereof; and copolymers of mixtures of one or more such butadienes-1,3, for example, of up to 75 weight percent of such mixtures of one or more mono-ethylenic compounds which contain a EQU CH.sub.2 .dbd.C.dbd. (IIA)
grouping, wherein at least one of the disconnected valences is attached to an electronegative group, that is, a group which substantially increases the electrical dissymmetry or polar character of the molecule.
Examples of compounds which contain the Formula (IIA) grouping and are copolymerizable with butadienes-1,3 are: the Formual (I) monomers, especially styrene; the unsaturated carboxylic acids and their esters, nitriles and amides, such as acrylic acid, methyl acrylate, ethyl acrylate, methyl methacrylate, acrylonitrile, methacrylonitrile, methacrylamide; vinylpyridines, such as 2-vinylpyridine, 2-methyl-5-vinylpyridine; methyl vinyl ketone, and methyl isopropenyl ketone--all of which besides those above mentioned in connection with the styrene polymers are also copolymerizable with styrene.
Examples of such conjugated diolefine polymer synthetic rubbers of polybutadiene, polyisoprene, butadiene/styrene copolymers and butadiene/acrylonitrile copolymers. The synthetic rubber may be solution-prepared or emulsion-prepared, be it a stereo-specific variety or otherwise.
Other conventional unsaturated sulfur-vulcanizable rubbers may also be used as the elastomeric constituent, such as "EPDM" a rubbery terpolymer of ethylene, propylene and a copolymerizable non-conjugated diene such as 1,4-hexadiene, dicyclopentadiene, dicyclooctadine, methylenenorbornene, ethylideneorbornene, tetrahydroindene, and the like. The analogous fluorocarbon, silicone and polysulfide rubbers may also be employed as an elastomer.
The styrene polymer-thermoplastic elastomer polyblends may be diblends, triblends or even blends of a greater number of constituents, including polyblend mixtures of one or more suitable TE's. Broadly speaking, the polyblends may be comprised of between about 40 and about 95 weight percent of the styrene polymer constituent. More often, however, the styrene polymer content ranges from about 50-85 weight percent, with polyblends wherein the proportion of styrene polymer is in the neighborhood of 80 weight percent being frequently preferred.
There are several known and heretofore disclosed and, to varying extents, employed to cross-link and/or improve the physical properties of styrene polymer-thermoplastic elastomer polyblend systems. These, all quite diversified but ordinarily and usually without significant modification(s) reasonably adaptable to ordinary blend processing (often involving melt conditions) procedures, include:
(1) The use of processing temperature (heat with oxygen, as from air, present) to effect cross-linking the elastomer in the polyblend system. The amount of cross-linking is affected by the mechanical temperature, speed generally in revolutions per minute, of the mixing heads and mixing time. In this technique, the following generalities are observable: PA1 (2) Using peroxide catalysts to cross-link the elastomer and improving blend properties. Cumene hydroperoxide; 1,1-bis(t-butyl peroxy)cyclohexane; and t-butyl hydroperoxide are effective for this. This sort of technology is disclosed in Refs. 3, 4, and 5. PA1 (3) Another known means is the use of beta radiation (as from an electron beam source) to cross-link the elastomer. This treatment of the prepared resin improves physical properties of the polyblend and appears optimum at about 1/2 megarad dosage.
Increasde of processing temperature PA2 Increase and mixing rate PA2 Increase of mixing time
Increased cross-linking PA3 Increased cross-linking PA3 Increased cross-linking
Nonetheless, nothing in prior art appears to realisitically concern itself with an improved and highly effective means and composing technique for greatly enhancing the important physical properties, especially the environmental stress crack resistance characteristics, by cross-linking effects in styrene polymer-thermoplastic elastomer polyblends to get better and more satisfactory products thereby and as a result thereof in the way so indigenously advantageous as in the present contribution to the art.